Conventionally available are optical disk players (optical recording and reproducing apparatus) that can read recorded information from optical disks (optical recording medium), for example, such as DVD (Digital Video Disc) and CD (Compact Disc). DVDs currently available in the market have the capacity as high as 4.7 GB, yet demand for higher density optical disks has been strong and there has been ongoing study for realizing such optical disks. It is well known that recording density can be effectively improved by using light of a shorter wavelength for the reproducing light, and/or by increasing the NA (Numerical Aperture) of the objective lens.
In one optical pickup currently available for reproducing information from a next-generation high-density optical disk, the numerical aperture (NA) of the objective lens has been increased from the conventional DVD's 0.6 to 0.85, and a wavelength of 405 nm, shorter than the conventional DVD's 650 nm, has been selected for the reproducing light, so as to reduce the size of an aperture spot and thereby increase recording density.
However, a problem of increasing the NA of the objective lens is that it drastically increases the coma aberration that is caused when the optical disk tilts, with the result that the focusing characteristic of the aperture spot may be impaired. Note that, as used herein, the term “coma aberration” refers to an aberration that is caused when the light is focused on an axis other than the optical axis. The coma aberration caused by a tilt of the optical disk is proportional to the thickness of the light transmissive layer, from the light incident face to the information recording face, of the optical disk. Accordingly, a proportional increase of coma aberration with an increased NA of the objective lens can be suppressed by reducing the thickness of the light transmissive layer of the optical disk. Based on this principle, it has been proposed to reduce the thickness of the light transmissive layer of the next-generation high-density optical disk to 0.1 mm from the conventional DVD's 0.6 mm.
At the same time, the next-generation high-density optical disk needs to provide compatibility with conventional DVDs and CDs, which are now widespread. That is, the optical disk player for reproducing the next-generation high-density optical disk is required to reproduce conventional DVDs and CDs as well.
However, this is faced with one problem; namely, compatibility with different kinds of optical disks is difficult to achieve when the wavelengths of light or thicknesses of the light transmissive layers are different between different kinds of optical disks. As a rule, the objective lens is designed for a specific thickness of the light transmissive layer of a particular type of optical disk, and a specific wavelength of light used therefor. Accordingly, in the event where the light transmissive layers of the optical disks have greatly different thicknesses or the optical disks use greatly different wavelengths, spherical aberration is caused on the aperture spot, impairing focusing characteristic of the aperture spot. Note that, as used herein, the term “spherical aberration” refers to the difference between a focal point for a paraxial ray near the center of the light beam and a focal point for a marginal ray distanced from the center of the light beam.
In view of this problem, there have been proposed optical pickups with a plurality of laser beam sources of different wavelengths and with a single objective lens, whereby a laser beam is converged on an information recording face with a required numerical aperture.
For example, Japanese Publication for Unexamined Patent Application No. 197717/2002 (published on Jul. 12, 2002) (Publication 1) discloses a technique using an optical system in which the objective lens is made with a diffracting face on a curved face of the objective lens, so as to record and reproduce information with respect to three kinds of optical disks having light transmissive layers of 0.6 mm, 0.6 mm, and 1.2 mm, for which the wavelengths of 400 nm, 650 nm, and 780 nm are used, respectively. The objective lens with the diffracting face is designed such that the first order component of the diffracted light is used for the light beam of each wavelength.
Another example is Japanese Publication for Unexamined Patent Application No. 306261/2000 (published on Nov. 2, 2000) (Publication 2), which discloses an optical pickup device including a first light source, a second light source, a focusing optical system, and a compensating optical system. In this optical pickup device, the first light source emits a light beam with a wavelength of 650 nm. The second light source emits a light beam with a wavelength of 780 nm. The converging optical system is configured to cause the light beam from the first light source to converge on an information recording face of a DVD without causing serious spherical aberration. The compensating optical system is disposed between the second light source and the focusing optical system. The compensating optical system is provided to suppress the spherical aberration that is caused when the focusing optical system focuses the light beam from the second light source focuses on an information recording face of a CD.
Yet another example is Japanese Publication for Unexamined Patent Application No. 93179/2001 (published on Apr. 6, 2001) (Publication 3), which discloses a technique concerning an optical pickup for reproducing optical disks having light transmissive layers of the same thickness, using different wavelengths of light. This technique uses two light sources for respectively emitting a light beam (blue light) of 405 nm wavelength and a light beam (red light) of 650 nm wavelength. A diffraction optical element and an objective lens that can focus the blue light on an optical disk having a light transmissive layer of 0.6 mm thick are also used. In this technique, the light of either wavelength is incident on the diffraction optical element as a parallel ray, and the second order component of the light diffracted by the diffraction optical element is used for the blue light, and the first order component of the light diffracted by the diffraction optical element is used for the red light, so as to attain sufficient diffraction efficiency for both of these different wavelengths of light, and, at the same time, compensate for spherical aberration generated in the red light.
Proceedings of the 63rd Annual Meeting of Applied Physics on “DVD/CD Compatibility Technique in Blue-ray Disc”, Naoki Kaiho et al., Fall, 2002, No. 3, P. 1008, Lecture Number (27p-YD-5) (Publication 4) discloses a technique using an optical system including an objective lens and a hologram (diffraction element) that serves as a concave lens only for a light beam of 785 nm wavelength, the optical system recording and reproducing information with respect to three kinds of optical disks having light transmissive layers of 0.1 mm, 0.6 mm, and 1.2 mm, for which the wavelengths of 405 nm, 655 nm, and 785 nm are used, respectively.
The following describes the problems that are caused when the technique disclosed in Publication 1 is used for optical disks with light transmissive layers of different thicknesses, including a next-generation high-density optical disk (λ=400 nm, light transmissive layer=0.1 mm), a DVD (λ=650 nm, light transmissive layer=0.6 mm), and a CD (λ=780 nm, light transmissive layer=1.2 mm).
As a rule, an optical pickup (compatible optical pickup) compatible with optical disks of different recording densities uses an objective lens for which aberration is compensated for with respect to the optical disk with the largest recording density. Therefore, the compatible optical pickup for the next-generation high-density optical disk, the conventional DVD, and the conventional CD uses an objective lens for which aberration is compensated for with respect to the next-generation high-density optical disk. The objective lens cannot be used directly for the DVD or CD whose light transmissive layers have different thicknesses from that of the next-generation high-density optical disk, because in this case spherical aberration increases to the level where recording or reproducing cannot be carried out.
One way to solve this problem when recording or reproducing DVD is to compensate for the spherical aberration caused by the thickness difference of the light transmissive layers, by generating aberration in the opposite direction. This can be carried out by causing the light beam to enter the objective lens as a diverging ray.
That is, in order to record or reproduce optical disks with light transmissive layers of different thicknesses, the light beams of the respective wavelengths are incident on the objective lens by varying the degree of convergence and/or divergence of each light beam.
When a parallel ray of blue light (λ=400 nm) is incident on an objective lens with an effective diameter of 3 mm to be focused on the next-generation optical disk with a light transmissive layer of 0.1 mm thick, the degree of divergence for the red light (λ=650 nm) needs to be about −0.03 in order to compensate for the spherical aberration caused by the thicker light transmissive layer of the DVD. Similarly, in this case, the degree of divergence for the infrared light (λ=780 nm) needs to be about −0.07 in order to compensate for the spherical aberration caused by the yet thicker light transmissive layer of the CD. Here, the degree of convergence or divergence is an inverse of a focal length, and the negative value indicates a diverging ray, and the positive value indicates a converging ray.
Here, the red light and infrared light, with their large degrees of divergence of incident ray on the objective lens, greatly impair the focusing characteristic of the light by causing coma aberration on the aperture spot on the optical disk when the objective lens shifts in the radial direction (direction substantially orthogonal to the optical axis of the incident light on the objective lens) during tracking or other operations. The impairment of focusing characteristic caused by radial shifting of the objective lens is more severe in CD because the degree of divergence for the incident light on the objective lens is greater in CD.
With the objective lens having the diffracting face as disclosed in Publication 1, attaining diffraction efficiency of 100% for the first order component of one wavelength limits the diffraction efficiency for the first order component of the diffracted light of the other wavelengths, with the result that a desired level of high diffraction efficiency cannot be obtained. This brings about the problem of poor light efficiency by a loss of light quantity. The loss of light quantity necessitates a laser beam of higher power for the recording of information in particular. Further, the diffracted rays of unnecessary orders may enter the detector as stray light when reproducing information, with the result that the signal may be impaired.
When the technique disclosed in Publication 2 is used for the optical pickup device for reproducing information from the next-generation high-density optical disk and the DVD, the optical pickup is provided with an objective lens with a large numerical aperture. The objective lens is made of glass of a high. refractive index, and therefore has strong wavelength dependency. The strong wavelength dependency of the objective lens poses a problem in that a focal point deviates greatly in the presence of wavelength fluctuations caused by mode hopping or high-frequency superimposition, which cannot be followed by an actuator.
When the technique disclosed in Publication 3 is used for the optical disks with light transmissive layers of different thicknesses (next-generation high-density optical disk with a 0.1 mm thick light transmissive layer, and conventional DVD, with a 0.6 mm thick light transmissive layer), and when the respective light beams of blue and red are incident on the diffraction optical element as parallel rays, the angle difference between the diffraction angle for the blue light and the diffraction angle for the red light, which is required to compensate for the spherical aberration caused by the large difference in thickness of the light transmissive layers, must be increased to about 2° to 3°. The angle difference is related to the pitch of the diffraction grating of the diffraction optical element, as shown by the graph of FIG. 35. It can be seen from FIG. 35 that the pitch of the diffraction grating needs to be as narrow as 3.5 μm to 4.5 μm in order to achieve the angle difference of about 2° to 3°.
Further, since the objective lens (infinite objective lens) is generally optimized for the blue light approaching from a point of infinity, the emergent ray from the diffraction optical element needs to be a parallel ray. That is, a ray of blue light that is bent on the diffracting face of the diffraction optical element needs to be refracted to a parallel ray on entering the refracting face (face of the diffraction optical element on the side of the objective lens). This is also effective in preventing aberration caused by misalignment of the diffraction optical element with the objective lens.
FIG. 36 represents a relationship between pitch of the diffraction grating and curvature of the refracting face of the diffraction optical element, when a parallel ray of blue light incident on the diffraction optical element emerges from the diffraction optical element as a parallel ray. Note that, the relationship represented in FIG. 36 is based on a diffraction optical element in an optical pickup using an objective lens with an effective radius of 2 mm. The refracting face of the diffraction optical element is spherical. It can be seen from FIG. 36 that the curvature radius of the refracting face of the diffraction optical element needs to be no greater than 2.2 mm in order to confine the pitch of the diffraction grating from 3.5 μm to 4.5 μm.
However, given the fact that the effective radius of the objective lens is 2 mm, and that the effective diameter of the diffraction optical element is also 2 mm, the refracting face with a curvature radius of no greater than 2.2 mm is substantially hemispherical, which is impossible to fabricate or practically useless. The refracting face may be made aspherical, but in this case the exceedingly small curvature makes fabrication of the diffraction optical element difficult. Even if it is possible to fabricate, the on-axis focusing characteristic is undesirably increased to 0.018 λ(rms) for all of the optical disks.
As a rule, the diffraction efficiency of a hologram (diffraction element) for a given wavelength is determined by the depth of the diffraction grating. FIG. 2 is a graph representing a relationship between depth of a diffraction grating and diffraction efficiency for different wavelengths of light of different diffraction orders. In FIG. 2, indicated by B0, B1, B2 are respectively diffraction efficiencies for the zeroth order, first order, and second order components of the diffracted light of a light beam of 400 nm wavelength for the next-generation high-density optical disk. Indicated by R0, R1 are respectively diffraction efficiencies for the zeroth order and first order components of the diffracted light of a light beam of 650 nm wavelength for DVD. Ir0 and Ir1 are respectively diffraction efficiencies for the zeroth order and first order components of the diffracted light of a light beam of a 780 nm wavelength for CD.
FIG. 2 represents one application of the technique of Publication 1 in the foregoing optical disk. As can be seen from the graph of FIG. 2 according to one embodiment of the present invention, when the depth of the diffraction grating is set such that the first order component of the diffracted light of 780 nm wavelength yields higher efficiency than the other diffraction orders of the light of this wavelength, the diffraction efficiency for the diffracted light of a predetermined diffraction order of the other wavelengths (zeroth order component of the diffracted light for the light beams of wavelength 405 nm and 650 nm) falls below about 10%. Conversely, when the depth of the diffraction grating is set such that the zeroth order component of the diffracted light for the light beams of wavelength 405 nm and 650 nm yield higher efficiency than the other diffraction orders of the respective wavelengths, the diffraction efficiency for the first order component of the diffracted light of 785 nm wavelength falls below about 10%. It is therefore practically impossible to set such a depth for the grating that the efficiency of light is increased for the all wavelengths of light.
More specifically, when the depth of the diffraction grating is set for the light beam of 405 nm wavelength, for which fabrication of a high power laser is difficult, so as to increase the diffraction efficiency, i.e., the efficiency of using light, to, for example, 80% or greater, the efficiency of using the first order component of the diffracted light of 780 nm wavelength decreases to 5% or less. In this case, the optical pickup is unable to produce sufficient light quantity for the recording or reproducing of information with respect to CD.